1. Field of the Invention
The present general inventive concept relates to an image forming apparatus and method thereof, and more particularly, to an image forming apparatus having an improved printing medium transferring method.
2. Description of the Related Art
In general, an image forming apparatus forms predetermined image information on a printing medium. The image forming apparatus may be an ink jet printer, or an electrophotographic laser printer for example.
The ink jet printer jets a tiny droplet of ink for printing ink onto a desired position on a printing medium to print an image. The ink jet printer may have a shuttle-type head in which a head moves right and left along a transverse direction of a transferring direction of the printing medium to print one line, or an array-type head in which nozzles are arranged along a width of the printing medium to print one line at a time.
The shuttle-type head ink jet printer generally has a desired speed profile graph of a printing medium according to time as illustrated in FIG. 1. That is, after the printing medium is unit-transferred during time T1 to T2 a predetermined amount in the transferring direction of the printing medium, the movement of the printing medium is paused during time T2 to T3. The head moves along the transverse direction to print one line during time T2 to T3. After printing one line, the printing medium is unit-transferred a predetermined amount, and another line is printed during time T4 to T5. The entire image of printed ink is formed by repeating this process. The term “unit-transfer” as used in this disclosure refers to an operation intended to achieve a transfer of a unit amount, such as an amount corresponding to a desired spacing between adjacent lines of printing of an image.
The ink jet printer may include a medium transferring unit to transfer the printing medium to the head in accordance with the desired speed profile in FIG. 1. The medium transferring unit may include a feeding roller and an idle roller pair to hold a leading edge part of the printing medium to transfer the printing medium to the head, and a driving roller and a driven roller pair to hold a trailing edge part of the printing medium to transfer the printing medium to the feeding roller/idle roller pair.
FIG. 2A is a graph illustrating a desired speed of a printing medium at a proximate point where the trailing edge part of the printing medium is separated from the driving roller/driven roller pair, and FIGS. 2B and 2C are graphs illustrating an actual speed and a position error of the printing medium, respectively. Here, a positive speed corresponds to a forward transferring direction and a negative speed corresponds to a backward direction or a direction opposite of the forward transferring direction. The maximum value of a position error value illustrated in FIG. 2C is approximately 0.45 mm, indicating that the printing is performed through unit-transferring the printing medium at an interval of 0.45 mm.
As illustrated in FIG. 2C, the position error can be seen as zero after the printing medium is unit-transferred during time T1 to T2 by an interval of 0.45 mm. As the head moves in the transverse direction during time T2 to T3, one line of image is printed. After this one line of image is printed, the printing medium is unit-transferred during time T3 to T4 and the head again moves during time T4 to T5 to print another line.
However, as illustrated in FIG. 2B, a sudden external disturbance is received by the ink jet printer during the printing at time T4 to T5. Although the printing medium is in the paused state and it is desired not to move the printing medium, the external disturbance generate a position error on the printing medium.
In particular, since the external disturbance arises during the printing process, there is no opportunity to compensate for the position error, and also the ink image line is formed in undesired position, thereby deteriorating image quality.